Inhibition of corrosive action of aqueous ammonium nitrate-urea on ferrous metals

ABSTRACT

Ferrous metal corrosion by an aqueous solution of ammonium nitrate and urea is inhibited by incorporating at least 0.02 percent P2O5, at least 30 percent of which is polymeric phosphate. Ammoniated superphosphoric acid is a source of the polymeric phosphates. The aqueous solution, which is useful as a soil fertilizer, has a pH of 6-7, contains at least 20 percent of each nitrogen ingredient, at least 15 percent water and up to 1 percent free ammonia.

United States Patent [72] Inventor Herbert Fleming Scott, Jr.

Prince George, Va.

[21] Appl. No. 730,897

[22] Filed May 21, 1968 [45] Patented Nov. 2, 1971 [7 3] Assignee AlliedChemical Corporation New York, N.Y.

[54] INHIBITION OF CORROSIVE ACTION OF AQUEOUS AMMONIUM NITRATE-UREA ONFERROUS METALS 1 Claim, No Drawings [52] US. Cl. 71/29, 71/30, 252/389[51] Int. Cl COSc 9/00 [50] Field ofSearch 71/28,29,

[56] References Cited UNITED STATES PATENTS 2,855,286 10/1958 Harvey71/29 3,067,024 12/1962 Boies et a1. 71/50 3,264,085 8/1966 Hignett eta1. 71/34 Primary Examiner-Reuben Friedman Assistant Examiner-Charles N.Hart ,7 Attorney.lohn J. Lipari INHIBITION OF CORROSIVE ACTION OFAQUEOUS AMMONIUM NITRATE-UREA ON FERROUS METALS BACKGROUND OF THEINVENTION This invention relates to aqueous ammonium nitrate-ureasolutions; in particular, it relates to the inhibition of the corrosiveaction thereof on ferrous metals.

Aqueous ammonium nitrate-urea solutions have had wide use for some timeas soil fertilizers. These solutions can be of any desiredconcentration; however, the solutions generally contain at least 20percent of each nitrogen-containing component and at least 15 percentwater. In some instances small amounts of free ammonia, say up to 1percent may be present.

Aqueous ammonium nitrate-urea solutions containing little or no freeammonia exert a marked corrosive affect on ferrous metals whichprecludes storage or shipping of the solutions in ferrous containers andhandling in ferrous lines, valves, and the like without damage to theferrous metal.

U.S. Pat. No. 2,855,286 relates to a process for protecting both ferrousand aluminum metals against corrosion by aqueous solutions of ammoniumnitrate containing little or no free ammonia, and which may contain ureaor sodium nitrate in addition to the ammonium nitrate. This patentdiscloses incorporating in the aqueous ammonium nitrate solution 0.02percent to 2 percent P in the form of a soluble phosphate to protectferrous and aluminum metals against corrosion. P 0 compounds mentionedinclude the free phosphoric acids (hydrogen phosphates) and thephosphate salts (as ortho, meta or pyro compounds) soluble to the degreerequired in the ammonium nitrate solutions.

Although the teachings of US. Pat. No. 2,855,286 are an importantadvance in the art, research in this field has continued in an effort tofind still better corrosion inhibitors. Ideally, a corrosion inhibitorshould substantially eliminate corrosion; if should be relatively low incost and should be effective at low concentration; it should be inliquid form for easy mixing with the fertilizer solution; and it shouldbe capable of being easily shipped and stored prior to its use.Inhibitors commonly used heretofore did not have these and otherrequirements which are satisfied by the inhibitors of this invention.

SUMMARY OF THE INVENTION In accordance with the instant invention,corrosion of ferrow metals, such as mild and carbon steel, by aqueoussolutions of ammonium nitrate and urea is effectively inhibited byincorporating in the solution a corrosion-inhibiting amount of P 0 atleast about 30 percent of which by weight is in the form of polymericphosphates. Ammoniated superphosphoric acid is the preferred source ofthis P P, and is normally added in an amount to provide at least about0.02 percent P 0, by weight.

Although the invention may be applied to aqueous ammonium nitrate-ureasolutions of any relative concentrations, with or without small amountsof free ammonia present, preferred solutions are those normally used assoil fertilizers or in the production of fertilizers, i.e thosesolutions of pH 67 containing on a weight basis at least about 20percent each of ammonium nitrate and urea, at least about percent ofwater, and O-l percent of free ammonia.

DETAILED DESCRIPTION OF THE INVENTION The P 0 of use in the instantinvention, which contains at least 30 percent by weight of polymericphosphates, is added to the aqueous solutions of ammonium nitrate andurea in the form of ammoniated superphosphoric acid. This lattermaterial can be produced by evaporating practically all the water fromaqueous orthophosphoric acid to form superphosphoric acid containing70-77 percent P 0 which is then ammoniated with aqueous ammonia. Theaqueous ammoniated superphosphoric acid will normally have the followingproperties: pH of from about 5 to 8, preferably about 6; a nitrogencontent of from about 9 percent to 12 percent by weight, preferably fromabout l0 percent to l 1 percent by weight and a total phosphorus contentmeasured as P 0 of from about 28 percent to 39 percent by weight,preferably about 34 percent to 38 percent by weight wherein polymericphosphates represent from about 30 percent to 70 percent by weight,preferably from about 50 percent to 65 percent by weight, based on theweight of the total phosphorus content. The polymeric phosphates includepyrophosphates v and polyphosphates. Of course, the use of ammoniatedsuperphosphoric acids with properties somewhat outside of these rangesis within the scope of the instant invention.

The ammoniated superphosphoric acid is added to the aqueous solution ofammonium nitrate and urea in an amount to provide an effectivecorrosion-inhibiting level of P 0, in the solution. Normally, a P 0level of at least about 0.02 percent by weight will be used althoughlower levels may be employed if less effective inhibition is acceptable.Levels as high as.

about 0.07 percent and even higher are effective, although levels inexcess of about 0.07 percent will usually not be used for economicreasons. Levels between about 0.03 percent and 0.04 percent arepreferred. These low levels of I50; containing the required polymericphosphates have been found to inhibit corrosion more effectively thanhigh levels of other materials used heretofore.

In addition to the high level of inhibition which is possible by itsuse, P 0, from ammoniated superphosphoric acid is advantageous forseveral other reasons. It does not significantly change the pH of theammonium nitrate-urea solution. which is preferably in the range of 6-7;a lower pH tends to intensify corrosion of carbon steel and a higher pHmay cause decomposition of the urea accompanied by loss of free ammonia.in addition, free ammonia may damage plants during direct application ofthe solution to the soil. Also, the inhibitor of this invention is nottoxic to plants or animals and is itself a plant food. Furthermore, thenovel inhibitor is not corrosive and may be shipped or stored in mildsteel prior to use.

A commercially available aqueous ammoniated superphosphoric acid havingthe following properties was found to be particularly useful forpurposes of the instant invention, and will be referred to as SolutionA" in the examples following hereinafter:

Nitrogen 10.11: by weight Phosphorus (as B0,) 34.2% by weight SulfateIon 1.7% by weight Trace Minerals Iron (Fe OQ ca. 1.0% by weight Calcium(CaO) ca. 0.1% by weight pH 6.0 Specific gravity at 60 F. L4 Salting outTemperature 0' F.

Distribution of Ammonium Phosphates Orthophosphate Polyphosphate 46% oftotal IEO; 541 of total P,O,

Eleven solutions of aqueous ammonium nitrate-urea were prepared based ona solution consisting of 45.1 percent ammonium nitrate, 34.8 percenturea and 20.1 percent water. 1nhibitors used were Solution A and reagentgrade H,P0,, weight percent.

Solution A Solution A solutions contained about 0.053 percent P 0 Thebottles containing the test solutions and specimens were capped andplaced side by side on a laboratory shelf for 30 days after which thespecimens were cleaned, dried, and weighed as in I"hibite d sohftionsfor each p were Prepared from the 5 example I. Average'corrosionpenetration rates were as folammonium nitrate-urea solution by admixingthe inhibitor just l prior to each series of corrosion tests.

The following examples are provided to more fully illustrate TABLE theinstant invention. They are provided for illustrative pur- 1 d the H d.t. th t. Solution Initial After test Penetration poses on y an are nocons ue as imi ing e inven ion, Number lnhibim, pH PH mm which isdefined by the appended claims.

3 Hapo ZE Q l0 Solution A 6.7 6.9 0.1

Specimens of carbon steel, ASTM A-285 Grade B, were ground to a 240 gritfinish, measured, cleaned with brush and Appearance of rusty Pmclpnatesolution 3 durmg the scouring powder, rinsed with water, rinsed withacetone, day contact time was further evidence of corrosion. Solutiondried, weighted and immersed in duplicate in Solutions 1, 2, 10 remainedClear and free of W P P Additional 1 1 4 5 and 5 at mom temperature theend f the Contact evidence of reaction between solution 3 and the steelis shown time with the Solutions, the specimens were cleaned with a bythe large shlflm P durmg the day period fiber brush and scouring powder,rinsed with water, rinsed A LE IV with e f n W ighted. Average corrosionAn apparatus consisting ofa stainless steel centrifugal pump penetrationrates in mils per year were calculated from the and a glass circulationloop with temperature controls was weight loss of the specimens. Table Ishows the relative corroconstructed for the purpose of determining thecorrosivity of siveness of these fertilizer solutions to carbon steel.arnmonium nitrate-urea fertilizer solutions at a flow velocity TABLE IContact Penetra- Equivalent time, Initial tlon, P10 wt. Inhibitor, wt.percent days pH m.p.y. percent Solution Number:

1 None 7 6.8 45.3 None 2.. H3PO4(85%)10-059.... 4 4.8 4. 8 0.036 4.-Solution A, 1.00 7 6.1 0. 2 0. 342 5.. Solution A, 0.50 7 a. 3 0. 30.171 6.. Solution A, 0.16 7 6.6 0.2 0. 055 11 Solution A, 0.07 14 3, 10. 024

EXAMPLE II of 6 feet per second. Carbon steel specimens were prepared inThe procedure for Example I was followed except that carbon steelspecimens were contacted with Solutions 1, 7, 8 and the same manner asdescribed in example 1 except that they were activated by contactingwith 18 percent HCl solution for 10 seconds, rinsed with water, andimmediately supported on 9 for 32 days at room temperature. Results wereas follows:

TABLE 11 Solution Solution A, Initial Penetration P,0, Number Weight pHmpy Weight 1 None 6.8 55.3 None EXAMPLE III- Two sets of duflica tecarboh steel specimens, prepared as described in example I wereactivated by contacting with 18 weight percent HCl solution for 10seconds, rinsed in water for 3 seconds, then one set was immersed inSolution 3 and the other set in solution 10. The average surface area ofeach specimen in solution 3 was 14.05 cm.*. In solution 10 the averagesurface area of each specimen was 14.07 cm. Both sion penetration ratessee table ivglass hangers and contacted with solution 1 in a long,straight, vertical, section of the circulation loop. The test apparatuswas then completely filled with Solution l and the pump was started.Contact was for 4 days at 6 feet per second and 34 C. This procedure wasrepeated for Solution 7. Average corro- I claim:

1. A process for inhibiting corrosion of ferrous metals contacted by aflowing aqueous solution of ammonium nitrate and urea having a velocityof at least about 6 feet per second, said solution containing about 45percent by weight ammonium nitrate and about 34 percent by weight urea,which comprises incorporating in said solution about 0.034 percent byweight of P 0 in the form of an aqueous ammoniated superphosphoric acidderived from wet-process phosphoric acid and having a 6.2 of about 6.2,a nitrogen content of about 10-] 1 percent by weight and a totalphosphorus content measured as P 0 of about 3438 percent by weight, saidtotal phosphates containing about 50-65 percent polymeric phosphates.

P0-W50 UNITED STATES PATENT OFFICE 5 69) CERTIFICATE OF CORRECTIONPatent No. 3,617,240 Dated Nmzember 2 J91] lnventofls) Herbert FlemingScott, Jr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 3, line 13, starting with "H P0 should appear in column 4, line12 in Table III.

Column 3, line 1?, "weighted" should be -weighed.

Column 3, line 21, "weighted" should be weighed.

Claim 1, line 66, column 4, "6.2 of about 6.2," should read pH of about6.2,-.

Signed and sealed this 13th day of April 1972.

Q EEAL A tte EDWARD PLFLETCHEHJR. ROBERT GOTTSCHALK Attesting Office].Commissioner of Patents

